The invention generally relates to an apparatus and method which achieve a bond between two or more materials of a composite structure, with differing thermal coefficients of expansion. In particular, the apparatus and method of the present invention produce a stress-free composite structure of silicon carbide and diamond.
High power RF transistor needs are increasingly being upgraded so that they are required to produce more power in less area. This places a greater emphasis on heat removal since these devices are made of semiconductor materials which exhibit degraded performance when their temperatures increase. In addition, many of the conventional high power RF devices are operated in a pulse mode which requires not only good thermal conductivity (for steady-state heat removal), but also requires very high thermal diffusivity to quickly spread the heat from the heat generating area. Table 1 shows the thermal properties of common semiconductor materials and other heat spreading materials.
TABLE 1 Heat Thermal Thermal Capacity Conductivity Diffusivity Density Material (J/gmc.sup.0) (w/cm.sup.0 K) (cm.sup.2 /sec) (g/cm.sup.3) GaAs 0.3 0.46 0.248 5.3 Si 0.7 1.5 0.93 2.3 SiC 0.628 4.5 2.23 3.2 Cu/W(15%) 0.29 2.55 0.54 16.4 Beryllium 1.05 0.25 0.084 2.85 Oxide Diamond 0.51 15 to 20 9.47 3.52
To achieve these higher powers and greater heat dissipation, the assembled transistor package must have a small thermal resistance between the actual power absorbing volume and the exterior surface in contact with some form of air flow cooling. This application would be applicable for any number of active or passive devices which generate or collect energy (heat). In addition, the material closest to the actual power absorbing volume should have a very high thermal diffusivity so that high transient thermal pulses can be spread rapidly away from the heat creating sites. A candidate to accomplish both of these is shown in FIG. 1.
FIG. 1 illustrates a SiC/diamond stack 100, which includes a diamond layer 102 with a thickness L, bonding material 104, with a thickness that is virtually zero, and a silicon carbide die 106 with heat creating sites 108 formed on an upper portion thereof. Although not required, in this example the silicon carbide die 106 has a thickness approximately equal to the diamond layer 102.
The thermal conductance of this SiC/Diamond stack 100 is 17 times better than a comparable Si/BeO stack. In addition, the diamond layer 102 provides a 4.2.times. improvement in thermal diffusivity over the SiC itself. The problem with such a solution is that the stack 100 cannot conventionally be bonded together. Stresses which result from the different coefficients of thermal expansion (CTE) either tear the bonding material 104 or fracture the stacking materials when cooled to operational temperatures. The bonding materials of choice are metals rather than epoxies because of the need for low thermal resistance. The metal chosen, for example, titanium or titanium nickel, must also have a solidification temperature higher than the highest operating temperature. The assembly of the stack 100 is thus done at temperatures up to 100.degree. C. higher than the upper operation temperature at the material interfaces. The maximum CTE stresses which result when cooled to the lowest operating temperature result in fracture or debonding. It is this problem that is addressed in this disclosure.